The present invention relates generally to shape memory and superelastic alloys and vacuum deposited metallic materials. More specifically, the present invention relates to nickel-based and titanium-based alloys fabricated by vacuum deposition technologies and which exhibit shape memory effect (SME) and/or superelastic behavior. The present invention also relates to a method of physical vapor deposition (PVD) of nickel-titanium alloys by sputter deposition in which several process parameters are controlled to achieve the inventive high-strength deposited nitinol films. It has been found desirable control the energy of particles emitted from the target as they arrive at the substrate surface. One process parameter, in particular, that has been found to contribute significantly to producing the inventive high-strength nitinol films is the mean free path of particles emitted during sputtering of the sputter deposition target must be greater than about one-half the distance of the spatial separation between the target and the substrate. This condition is set by the requirement that the kinetic energy of the particles that are deposited must be much greater than the thermal energy in order to promote non-columnar film growth. This condition of the arrival of energetic particles to the substrate can be fulfilled also in other deposition methods, such as ion beam deposition and laser ablation, for example, hence the inventive method can be practiced by using other methods than sputtering. Control of other deposition process parameters has also been found to influence the strength characteristics of the resulting deposited film, including, employing a hollow cathode dc magnetron, controlled heating of the target, controlling the base pressure and working gas quality to avoid oxygen contamination of the depositing film, controlling the deposition pressure, controlling the surface roughness of the deposition substrate, controlling the composition of the substrate to avoid diffusion contamination into the depositing nitinol, and applying a negative bias voltage to the substrate.
In the metallurgic arts, it is known that nickel-titanium alloys having nearly stoichiometric 50-50 atomic percent nickel and titanium exhibit SME and are superelastic above certain temperatures. It is also known that ternary alloys that contain mostly nickel and titanium but also contain other components such as copper, chromium, tantalum, zirconium, or other metals also often exhibit SME. Similarly, nickel and titanium-based quaternary or more complex alloys can exhibit the SME. As used in the art, and as used in this application, the term “nickel-titanium alloy” is intended to include binary, ternary and quaternary alloys containing nickel and titanium that exhibit shape memory effect.
SME nitinol alloys may be manufactured both by conventional metallurgy and by vacuum deposition (See U.S. Pat. No. 5,061,914). It has been found that vacuum deposition fabrication offers the possible advantage of readily adding alloying elements to produce a large variety of alloy films having a wide variety of transition temperatures. Heretofore, however, vacuum deposited films have exhibited inferior mechanical properties when compared with similar articles fabricated by conventional metallurgy. For purposes of this application, those materials fabricated by conventional metallurgical methods will be referred to as “wrought nitinol” or “wrought nickel-titanium alloys.” This difference in mechanical properties between wrought nitinol materials and vacuum deposited nitinol materials significantly limits the usefulness of vacuum deposited nitinol films. Some of the most appealing potential applications of nitinol films include micro-electro-mechanical (MEMS) devices and medical devices, such as endoluminal stents. However, since its inception over ten years ago, virtually no vacuum deposited thin film nitinol devices have been commercially marketed because of their insufficient strength relative to similar devices fabricated from wrought nitinol.
Shape memory and superelastic nitinol materials undergo a reversible phase transition between martensitic (M) and austenitic (A) phases. It is this property that makes use of nitinol materials especially desirable in certain applications, including medical devices, microelectronic sensor devices or the like. The M phase is stable at lower temperatures and/or high stresses, and the A phase is stable at higher temperatures and/or lower stresses. One of the most important characteristics of nitinol is the M to A transition temperature. This transition occurs within a range between As (start) and Af (finish). The transition process is endothermic and may be characterized by the heat effect, ΔH and by the Ap temperature where, for a given rate of heating, the transition heat effect is maximum.
Wrought nitinol is produced by vacuum melting ingots. This method results in a nitinol material having the same transition temperature as the nitinol ingot, termed the “ingot Ap.” The ingot Ap depends on the stoichiometry of nickel and titanium and the ingot, and may be between −50° C. and 100° C. The ingot Ap is lower where there is excess nickel, i.e., greater than 50 at. %, and higher when the alloy contains excess titanium, i.e., more than 50 at. % Ti. Ingots are processed by shaping into sheets or tubes for further use in the industries such as the medical device industry. Particular end applications of nitinol materials require particular transition temperature values. However, ingots with a range of different Ap values are not available for each application. In order to address this difficulty in available ingots, a method called precipitation annealing is employed in order to adjust the Ap value of a given product.
Precipitation annealing typically involves annealing nitinol at temperatures between 200-500° C. for 10-180 min., then allowing the material to cool at a controlled cooling rate or by quenching at a temperature below about 200° C. As a result of precipitation annealing, the excess component in the ingot, either Ni or Ti, will precipitate out from the crystal structure and form inclusions such as, for example, Ni3Ti2, NiTi2, or the like. These inclusions constitute a separate phase along the nitinol grain boundaries or within the nitinol grains and are termed “precipitates.”
The degree to which precipitate formation is necessary depends upon the relationship between the ingot Ap and the desired device Ap. Currently, manufacturer of a particular nitinol device, such a medical device manufacturer, e.g., a stent manufacturer, will purchase raw material, such as sheets or tubes from an nitinol ingot fabricator. The ingots from which the raw material is fabricated, will, in all likelihood vary in their material constitution and Ap value during the life of the device manufacturer's product line. Thus, in order to achieve a desired device Ap, device manufacturers must adjust the precipitation-annealing step for the given ingot Ap of any particular batch of raw material received from the ingot fabricator. The inevitable result of this need to engage in precipitation annealing to adjust for the ingot Ap is that there is variability in the amount of precipitates within the same device product line depending on the starting raw material. Because control of the ingot Ap values ingot to ingot is extremely difficult, this variability is present even where ingots are made by the same fabricator.
Precipitates in metals have implications for the mechanical, corrosion, and fatigue properties. Precipitates tend to constrain slip plane movements during plastic deformation and hence the concept of precipitation hardening of metals. Hardening, if it is desirable, can also be achieved using cold working in traditional metallurgy. In the case of thin film metallurgy, hardening may be controlled by the deposition parameters through controlling grain size, which is a function of substrate temperature and growth rate, among others. So, even if hardening is desirable, it can be achieved by alternative methods, without resorting to induced precipitate formation. Concerning other mechanical properties, precipitates tend to make materials more brittle, and decrease fatigue life. This is caused by local strain fields that arise around the precipitates, which are situated incongruently between the grains of the material. Precipitates may initiate micro-cracks along the grain boundaries and are known to contribute to intergranular failure. With regard to corrosion properties, precipitates may have detrimental effects in two ways: (i) the mentioned strain field and the related micro-cracks may increase the effective surface area exposed to corrosive environment and (ii) local micro-elements (corrosion pairs) may be formed by the precipitate and the surrounding nitinol matrix.
It is known that nitinol can be made not only using the traditional metallurgical approach described in the previous paragraphs but also using film deposition technologies. Inherent in this approach, the producer of nitinol can have as good or better a control of the Ap transition temperature than in the production of ingots. Deposition can provide fine adjustment of the nitinol chemical composition and thus to eliminate or reduce precipitation thereby reducing hardness, and consequently reducing the plateau stress, and improving the strength at the same time. However, deposition technologies are not commonly used in applications where the composition control must be within about 0.1 at % in order to control the transition temperature within about 10° C.
The most common deposition method for producing nitinol is dc sputter deposition. We will describe the invention in terms of a distinct form of dc sputter deposition, i.e., using the example of hollow cathode (HC) dc magnetron sputtering (See, e.g., E. Kay, Cylindrical Cathode Sputtering Apparatus Including Means for Establishing a Quadrupole Magnetic Field Transverse of the Discharge, U.S. Pat. No. 3,354,074, Sep. 16, 1963). However, the skilled in the deposition art will see that the principles outlined are applicable for a wide range of deposition methods.
Typically, sputter deposited nitinol is more Ni-rich than the sputtering target employed. The reasons for this are complex and without a good understanding of these reasons, various researchers have employed a variety of remedies. These include (i) the addition of extra Ti to the target in form of Ti sheets placed on the target, or some similar approach, and (ii) allowing the target to reach high temperatures, whereby, as experience shows, the Ti content of the films is enhanced. It is an object of the present invention to provide a method of adjusting the Ti content (Ni to Ti atomic ratio) of deposited nitinol films using the above and the adjustment of sputtering parameters.
Vacuum deposition of nitinol provides the additional advantage over wrought nitinol in that high Ni content material can easily be produced. The manufacturing of high Ni content wrought material is impeded by its extreme abrasiveness and toughness that makes extrusion, rolling and the like not feasible. In particular, the production of devices from wrought nitinol with Ap<−20° C. is very difficult if not impossible. (See, e.g., D. Hodgson and S. Russell: “Nitinol melting, manufacture and fabrication” in Minimally Invasive Therapy & Allied Technologies, Vol. 9 No 2 March 2000 pp 61-65). Further advantages of vacuum deposition include (1) the ability of making thin walled tubes with high wall thickness uniformity, (2) the ability of making thin sheets with high thickness uniformity, (3) the ability of making objects with complex shapes, for example tubes with variable diameter along the length, such as funnel and balloon shapes, (4) better control over the material purity (5) control over material composition in terms of adding minor alloying elements such as, for example Ta in order to improve radio opacity.
Typically, sputter deposited nitinol has inferior mechanical properties as compared to wrought nitinol. This inferiority is manifest most clearly in the ultimate strength. The ultimate strength of the nitinol material described by Johnson (A. D. Johnson et al. US Patent Application 2001/0039449) appears from FIG. 3 in the published application to be approximately 500 MPa.
Mechanical properties of metals typically depend on their microstructure. Specifically, microstructure of deposited metal films consists of two main types of features: (i) the grain structure with the grain boundaries and precipitates both intragranular and intergranular, and (ii) texture such as columns. The grain structure is important because it determines the fundamental mechanical properties. Amorphous metals, i.e., those metals having no defined grain structures or grains that are too small to be detected by X-ray diffraction, are known to be very hard and brittle. Similarly, crystalline metals having very small grain sizes are also known to be quite brittle, but become more ductile as the grain size increases. When the grain size becomes very large, the metals lack strength and have low elastic limits. Thus, achieving a correct grain size plays a significant aspect of any metal fabrication technology. It is known that for wrought nitinol materials it is desirable to have grain sizes typically within the range of 0.1-10 μm in order to have practical mechanical properties.
In order to deposit nitinol films having thicknesses on the order of 0.1-25 μm, high deposition rates are typically employed. At such higher deposition rates, the resultant nitinol film develops generally columnar grain morphology. This columnar morphology is significant because it imparts features similar to grain boundaries, yet the columnar features traverse substantially the entire thickness of the resulting film. Like typical grain boundaries, the columnar grain morphology creates defect regions that are weaker than other regions of the film, and contaminants and precipitates may segregate into these defect regions. It is necessary, therefore, to avoid columnar grain growth during vacuum deposition in order to obtain nitinol films with higher mechanical strengths. Columnar growth results from the combination of the following factors: (i) low surface diffusion rate, (ii) expressed surface features, such as roughness, and (iii) directional deposition See, e.g., G. S. Bales, R. Bruinsma, E. A. Eklund, R. P. U. Karunasiri, J. Rudnick, A. Zangwill, Growth and Erosion of Thin Solid Films, Science, Vol. 249 p. 264-268, July 1990). It is an object of this invention to provide a method of choosing deposition parameters that result in nitinol film growth process without columnar growth.